In vehicle air-conditioning systems installed in electric vehicles, since an internal combustion engine is not installed in the vehicles, heating cannot be performed by using coolant or combustion exhaust heat. Thus, in general, a heat pump cycle using an electric compressor is adopted for an electric-vehicle air-conditioning system. The biggest challenge of the heat pump cycle is that heating becomes difficult or the heating capacity becomes insufficient when the outside-air temperature is low or at the operation starting time (heating start-up time), which is well known.
Thus, various improvements have been conventionally proposed. As a heat source for heating, it is conceivable not only to use exhaust heat from a drive motor that includes an inverter and exhaust heat from a battery but also to use an electric heater or a combustion heater together with such exhaust heat because, with such exhaust heat alone, the capacity is insufficient or it takes time to use the exhaust heat (for example, see PTLs 1 and 2).